Many industrial applications require processing in a vacuum, from a ultra high vacuum (“UHV”), that is, a very low gas pressure of less than about 10−8 Torr, to a rough vacuum, in which the pressure can be almost as high as atmospheric pressure. The terms “vacuum” and “pressure” are used in opposite senses, in that a “high vacuum” means a “low pressure” and vice versa.
Different types of vacuum gauges are used to measure vacuum in different pressure ranges. For example, a Pirani gauge is suitable for use in a vacuum from 10−5 Torr (1.3×10−3 Pa) to atmospheric pressure; a hot cathode ion gauge (HCIG), is suitable for vacuums from about 10−2 (1.3 Pa) to about 10−10 Torr (1.3×10−8 Pa); and the cold cathode ion gauge (CCIG), is used for vacuum from about 10−2 Torr (1.3 Pa) to about 10−13 Torr (1.3×10−11 Pa). Each type of gauge has advantages and disadvantages.
The Pirani gauge determines pressure by measuring thermal conductivity, which varies with the gas pressure in the vacuum chamber. A Pirani gauge can effectively measure pressure from a low vacuum up to atmosphere, but loses sensitivity in a high vacuum. In HCIGs, thermionically emitted electrons are accelerated and ionize gas molecules, which support a discharge current that varies with the gas pressure. The discharge current is correlated to pressure to measure the vacuum. An HCIG can measure a wide range of vacuums, but the hot filament produces heat, which can cause gases in the vacuum chamber to chemically react to produce compounds that promote contamination and corrosion. The heated filament also “outgases,” that is, drives off gases from the filament into the vacuum chamber. In contrast, a CCIG uses a high voltage static electric field and an orthogonal magnetic field to enhance the gas molecule ionization process. The CCIG reduces or eliminates heat-related failures.
FIGS. 1A and 1B show schematically a typical prior art standard CCIG 100, in which a high voltage source 102 supplies a constant high voltage, typically between about 2 kV and about 6 kV, between an anode 104 and a cathode 106 to produce an electric field 108 in the space between the anode and cathode. A magnetic field 110 orthogonal to the electric field is concurrently applied. The magnetic field, which produces a force on charged particles perpendicular to their velocities, causes the charged particles to traverse a meandering path, which increases the probability of collisions with gases in the gauge. Starting from the first stray charged particle, a stable discharge current builds up through various collisions between charged particles and stray gases.
FIG. 1B shows a typical collision sequence in which an electron 120 collides with a gas molecule to generate an ion 122 and additional electrons 124. Ion 122 also generates an electron 126 when it impacts the cathode 106. The movement of charged particles to the anode and cathode constitutes a current, referred to as an “ionization current” or a “discharge current,” which is measured by a current meter 112. The discharge current depends sensitively, typically exponentially, on the level of vacuum. At higher vacuums, there are less gas molecules to collide with and generate charges. As the vacuum changes from low to high, the discharge current decreases. The discharge or ionization current of the CCIG is detected, amplified, and used for calibrating the pressure in the vacuum. The discharge current also depends on the type of gases in the vacuum chamber.
In practice, CCIGs are used mainly for measuring vacuums higher than about 10−5 Torr. In a lower vacuum range from about 10−4 Torr to about 10−2 Torr, the greater concentration of gas molecules creates a very large ionization current, which causes excessive heat and sputtering of materials from the electrodes. Attempts have been made to solve the heating and sputtering problem to extend the CCIG measuring range to lower vacuum between about 10−4 Torr and about 10−2 Torr. For example, Klemperer describes in British Patent No. 555 134 for “Improvements in or relating to apparatus for measuring low gas pressure” a CCIG operating in two modes: a constant voltage operation in the high vacuum range at which discharge current is limited by the low gas pressure, and a constant current operation mode for operation in a low vacuum range to reduce the discharge current. U.S. Pat. No. 4,967,157 for a “Method And Circuit For Extending The Range Of A Cold Cathode Discharge Vacuum Gauge” to Peacock describes adding a large series resistor to the anode circuit to reduce the excessive current in low vacuum applications.
While known solutions exist for using CCIGs at low vacuum, in most applications CCIGs operate reliably in a high vacuum with a long lifetime. In some high vacuum applications, however, CCIGs tend to fail after a relatively short period of use. For example, charged particle beam systems, such focused ion beam systems and electron beam systems, used for applications such as circuit edit and photolithography mask repair, often employ metal organic gases and other unfriendly gases for milling and deposition. Although the gas pressure in such application is sufficiently low to avoid excessive heat and sputtering, CCIGs in such applications can still suffer a short average lifetime of about two weeks. Typical failures, which can occur after between two weeks and a few months of operation, are manifested as questionable vacuum readings, unstable vacuum readings, and malfunctions of the gauge.
Prior art efforts to increase the useful life of CCIGs were directed to solving the heating and sputtering problem of CCIGs when used in low vacuums, from about 10−4 Torr to about 10−2 Torr. At higher vacuums, the discharge current is relatively low, so heating and sputtering problems do not occur. The problem of short CCIG lifetimes when operating at high vacuum, such as pressures less than about 10−4 Torr, less than about 10−5 Torr or less than about 10−6 Torr, has not been solved because the mechanism for failure at the low pressures was unknown.